1. Field of the Invention
The invention provides pigment-containing polycarbonate compounds having improved dispersion of the pigment particles in the polymer matrix, and a process for the preparation of these compounds. Carbon black is preferably used as the pigment, “carbon black” in the present invention representing all particulate pure carbon substrates and carbon compounds, for example colour carbon blacks, conductivity carbon blacks, carbon nanotubes, graphite. The pigment-containing polycarbonate compounds can contain further polymers, such as, for example, elastomers or graft polymers, or further thermoplastics, such as, for example, polyesters.
The present invention relates further to the use of pigment masterbatches containing the pigment and a demoulding agent which is to be added to the polycarbonate composition.
The present invention relates further to a process for the preparation of such polycarbonate compounds having improved dispersion of the pigment particles in the polymer matrix, in which, in the compounding of the polycarbonate composition, a masterbatch of the pigment in fatty acid esters based on aliphatic alcohols or polyols is used. The invention further provides the preparation of such pigment masterbatches in fatty acid esters.
2. Description of Related Art
A technical problem when incorporating pigments, and carbon black particles in particular, into thermoplastic polymer compositions is that of dispersing the pigment particles completely and uniformly in the polymer matrix. Incompletely dispersed pigment particles form pigment agglomerates which apart from colour inhomogeneities and inadequate depth of colour also result in particular in defects which have an adverse effect on the mechanical properties of the polymer compositions, such as their strength and ultimate elongation, and also on the surface properties of the materials. Larger pigment agglomerates lead, for example, to faults and defects on the surface of such compositions such as pitting, streakiness and, ultimately, to an undesirable reduction in the degree of gloss. In a composite with other materials, such surface defects can additionally also adversely affect the composite adhesion properties (for example lacquer adhesion).
Carbon-based pigments—such as, for example, carbon blacks, graphites, fullerenes, graphenes, activated charcoals and carbon nanotubes, which are used in many commercial fields of application, for example for black colouration, for increasing the electrical or thermal conductivity of the composition, for mechanical strengthening or also for binding and reducing the volatility of low molecular weight organic compounds such as residual monomers or odour-bearing substances are distinguished by particularly strong interparticle binding forces and therefore have a particularly high tendency to form agglomerates, which can be broken up again only with difficulty on incorporation into thermoplastic polymers.
Various methods are known from the prior art for improving the dispersion of such pigments in thermoplastic polymer compositions. For example, pigment dispersion can be improved by increasing the specific energy input by means of shear during the incorporation of the pigments into the polymer melt in conventional compounding units such as twin-screw extruders or internal kneaders.
However, the energy input which can be used for pigment dispersion is technically limited in the case of polymer melts, in particular those having a low viscosity, that is to say high melt flowability, as is required for good thermoplastic processability in most fields of application. In other cases, the energy input is limited by the thermal loading capacity of the polymer melt into which the pigment is to be incorporated. High specific energy inputs naturally lead to high process temperatures which, depending on the polymer, can lead to undesirable damage, ageing or even decomposition of the polymer.
A further method is the use of a highly concentrated masterbatch of the pigment in a polymer matrix, but the technically achievable concentration of the pigment in the polymer matrix is not high enough for an economic application without the use of further additives/processing aids. Furthermore, good pigment dispersion in the end product can be achieved with this method only if the pigments are already well dispersed in the masterbatch, which is only insufficiently ensured when using polymer matrices, in particular in polycarbonate.
A further possibility for improving the dispersion of pigments consists in using dispersing aids, which reduce the intermolecular interactions between the individual pigment particles or pigment aggregates within a pigment agglomerate and thereby facilitate the breaking up of the agglomerates during the preparation of the compounds. The disadvantage of the use of such dispersing aids, which have no other necessary action in the composition, is that they remain in the polymer composition that is produced and, as a result, may possibly adversely affect the application-related properties of the target products.
For example, such dispersing aids in multiphase compositions (blends) of different polymers (such as, for example, impact-modified polymers) can adversely affect the phase compatibility of the different polymer components by accumulating at the phase boundaries and thereby adversely affect the mechanical properties of the blend composition. Likewise, these additives can catalyse undesirable ageing processes in certain polymer systems, for example hydrolytic decomposition reactions in polycondensation polymers.
The preparation of pigment concentrates in wax-like compounds is already known from U.S. Pat. No. 4,484,952, wherein the preparation of carbon black concentrates in PETS (pentaerythritol tetrastearate) is also described. However, the shear forces which occur under the stirring, spraying or centrifugation conditions mentioned in U.S. Pat. No. 4,484,952 for mixing the pigments with the carrier are too small to achieve sufficiently fine separation and uniform distribution of the pigments in the carrier material in the case of highly agglomerated pigments. However, this is a necessary requirement for subsequent uniform dispersion of the pigments in a polymer matrix with the aid of such pigment concentrates. Moreover, U.S. Pat. No. 4,484,952 gives no indication of the quality of the pigment dispersion which can be achieved in thermoplastics with carbon black concentrates so prepared, in particular the dispersion of the carbon black which can be achieved in polycarbonate compositions. Furthermore, there is no information in U.S. Pat. No. 4,484,952 regarding the process parameters used in the preparation of the pigment concentrates and the energy input as well as the mixing unit used, which have a critical influence on the quality of the dispersion.
The preparation of pigment and, in particular, carbon black concentrates in wax-like compounds is also known from U.S. Pat. No. 4,310,483. However, this is likewise a concentrate form in which only a low energy input for the separation of agglomerated pigments and their uniform distribution in the matrix material occurs. The preparation process is in fact aimed at improving the metering properties of the described pigment concentrates, dust formation being largely avoided and a more advantageous metering form being achieved by wetting of the pigments. The amount of pigment in the described carbon black concentrates far exceeds the amount of granulating aid used. Regarding the quality of the pigment dispersion in thermoplastics using such pigment concentrates, it is stated in U.S. Pat. No. 4,310,483 that it is equally as good as in the case of the metering of pure powder without the use of granulating aids, but an improvement in the dispersion is not described.
WO 2002/092702 relates to the coating of carbon black pellets by spraying with wax-like compounds, accordingly, for example, also with PETS, in order to improve the metering properties of carbon black products by the coating.
The preparation of carbon black-containing polycarbonate moulding compositions using carbon black masterbatches is described in EP 578 245 A2. However, the masterbatches here are masterbatches in polyethylenes. Polyethylenes lead to disadvantageous property changes in polycarbonate moulding compositions, for example in respect of the low-temperature strength of the moulding compositions, and are therefore to be avoided.
US 2009/0057621 A1 describes the melt-mixing of carbon-containing thermoplastic masterbatches with thermoplastics without isolation of the masterbatch but with simultaneous continuous metering into a second thermoplastic melt, wherein the thermoplastic can also be polycarbonate. Such a process is technically too complex and inflexible, however.